. >> reporter: hitting the fast food drive-in for science. >> can i get five soft tacos. >> can i get two sausage burritos? can i get a number four? >> reporter: over and over and over again. it's been dave giocolo's life the past three months or so. as has been eating the food. a precisely measured 1,000 calories a day -- an extra meal daily -- fast food only. how many calories? >> 770. >> reporter: why? because this man is paying him to. dr. samuel klein is a researcher at washington university medical school in st. louis, trying to understand why weight gain leads to diabetes and hypertension. and how that relates to levels of fat in the liver and muscles. at some point, research on rats alone just isn't enough. somebody has to eat this food this way to -- >> ultimately, it has to come to people. >> reporter: and a radio ad -- >> attention overweight volunteers -- >> reporter: -- that offered a cash incentive, up to $3,500. depending on how long it would take. >> once i got into work, i called them right away. >> reporter: so did nurse dawn freeman. >> it probably took a month to j